1. Field of the Invention
The present invention relates to a process for preparing diaryl carbonates by reaction of an aromatic hydroxy compound (e.g. phenol) with carbon monoxide and oxygen at elevated temperature in the presence of a base, a quaternary salt, a dessicant, a catalyst and a cocatalyst, which is characterized in that the catalyst is activated prior to the reaction. In a particular process variant, the base used is a preformed alkali metal phenoxide.
2. Description of the Related Art
It is known that aromatic carbonates can be prepared by oxidative reaction of an aromatic hydroxy compound with carbon monoxide in the presence of a homogeneous noble metal catalyst (German Offenlegungsschrift 28 15 512). The noble metals proposed are the elements of group VIIIb, with a preference being given to using palladium. During the reaction, this palladium(II) species is reduced to palladium(0) and reoxidized by oxygen with the aid of a cocatalyst to give palladium(II) again. Cocatalysts which can be used are, for example, various manganese or cobalt salts in different oxidation states. Besides these cocatalysts, use is made of a base, a phase-transfer catalyst and a dessicant. Methylene chloride is preferably used as solvent. According to German Offenlegungsschrift 27 38 437, sterically hindered tertiary amines are used as base and a molecular sieve is used as dessicant.
Disadvantages of these processes are, besides the use as solvent of the toxic, volatile methylene chloride which requires a high level of safety precautions and has to be recovered at considerable cost, long reaction times and the poor space-time yields associated therewith. However, for an industrial reaction, the insufficient reproducibility proves to be the actually decisive disadvantage, since the same procedure can give, from batch to batch, completely different results, even complete failure of the catalysis.
The high price and the oxidative instability of the sterically hindered tertiary amine bases proposed in German Offenlegungsschrift 27 38 437 is a further disadvantage of this process. Recovery of the base is technically complicated. In addition, a considerable part of the base is decomposed during the long reaction times, so that large amounts of the expensive base continually have to be replaced, which makes an economical utilization of the process difficult.
J. E. Hallgren and G. M. Lucas in Journal of Organometallic Chemistry 212 (1981) 135-139 report the use of aqueous sodium hydroxide solution as base. In the presence of small amounts of 50% strength aqueous sodium hydroxide solution and a phase-transfer catalyst, Hallgren and Lucas observe an increase in the reaction rate, compared with the use of tertiary amines. A considerable disadvantage of this procedure is the fact that aromatic carbonates, which are to be prepared by this process, are rapidly decomposed by aqueous sodium hydroxide solution (Ullmanns Encyclopa/ die, 5th edition, vol. A5, pp. 197-202). The cleavage reaction of aromatic carbonates proceeds so quickly in the presence of even catalytic amounts of aqueous sodium hydroxide solution that only low carbonate concentrations can be achieved in the reaction system. In addition, in this case too, the toxic, volatile methylene chloride is used as solvent, which brings with it the above described problems. Furthermore, the simultaneous presence of methylene chloride and sodium hydroxide results in a particular danger, since these, as is known to those skilled in the art, react to give highly reactive dichlorocarbene which leads to secondary reactions, and may even react spontaneously and explosively. Owing to this uncontrollable reaction, industrial use of this process is not possible either. In addition, these measures do not improve the reproducibility.
European Patent Specification 503 581 proposes the use of various copper salts as cocatalyst. Besides this cocatalyst, the use of considerable amounts of various quinones/hydroquinones as electron-transfer catalyst is also proposed. These measures do not improve the reproducibility. The industrial use of this process is therefore not possible either. In addition, this process likewise uses methylene chloride as solvent. The separation of the electron-transfer catalyst from the reaction mixture requires considerable additional expense in this process. Furthermore, hydroquinones are aromatic bifunctional hydroxy compounds which can be converted into carbonates in the same manner as phenol. The separation of the byproducts formed in this way can only be achieved at great expense. Recovery of the electron-transfer catalyst used is thus not possible. The formation of the byproducts would, at a given reproducibility, considerably lower the selectivity and thus the economics of this process.
The application WO 93/03000 describes a process for preparing aromatic carbonates which does omit the use of a solvent, but still requires considerable amounts of cocatalyst and electron-transfer catalyst. This too does not solve the problem of the insufficient reproducibility, so that all in all an industrially useable process has not hitherto been available.
It is therefore an object of the invention to find a process which allows the synthesis of aromatic carbonates to be carried out reproducibly under industrially achieveable conditions.